Alginates are hydrophilic marine biopolymers with the unique ability to form heat-stable gels that can develop and set at physiologically relevant temperatures.
Alginates are a family of non-branched binary copolymers of 1-4 glycosidically linked β-D-mannuronic acid (M) and α-L-guluronic acid (G) residues. The relative amount of the two uronic acid monomers and their sequential arrangement along the polymer chain vary widely, depending on the origin of the alginate. Alginate is the structural polymer in marine brown algae such as Laminaria hyperborea, Macrocystis pyrifera, Lessonia nigrescens and Ascophyllum nodosum. Alginate is also produced by certain bacteria such as Pseudomonas aeruginosa, Azotobacter vinelandii and Pseudomonas fluorescens (WO04011628 A1).
Alginate gels are produced when a divalent cation forms ionic bonds with the negatively charged group from a G residue from each of two different alginate polymers, thereby cross-linking the two polymers. The formation of multiple cross-linkages among numerous alginate polymers results in the matrix that is the alginate gel structure.
Alginate gels can be hydrogels, i.e. cross-linked alginate polymers that contain large amounts of water without dissolution. Biopolymer gels, such as alginate hydrogels are attractive candidates for tissue engineering and other biomedical applications. Because of this and the ability to form gels under physiologic conditions, alginates are widely used and studied for encapsulation purposes and as a biostructure material. The entrapment of cells in alginate beads is a commonly used technique. Also alginates have been shown to be useful material for other types of biostructures, including tissue engineering applications and as scaffolds for nerve regenerations.
Different methods for making alginate hydrogels exist. The most common method is the dialysis/diffusion method where the alginate solution is gelled by diffusion of gelling ions from an outer reservoir. This method is mostly used when making alginate gel beads and in food applications. The manufacturing of alginate microbeads is a rapid process limited by the diffusion of gelling ions into the gel network. Although this process is well suitable for entrapment of cells in microbeads, it is less useful for the production of other shapes or structures. For manufacturing of gel structures of larger size diffusion gelling systems may have limited possibility. This is because the rapid gelling process limits the time to allow shaping of the gel structure.
A delay in the gelling process may be used to allow for the injection of solutions into the body and/or to mix cells or other biomaterial into the gel matrix prior to the gel forming. Therefore, alternative methods have been developed for the manufacturing of other types of biocompatible alginate gel structures. The gelling speed may be reduced by using internal gelling systems of which the gelling ions are released more slowly inside the forming gel. This is described as internal setting of the gel. Commonly, in an internal gelling system, a calcium salt with limited solubility, or complexed Ca2+ ions, are mixed with an alginate solution into which the calcium ions are slowly released. Calcium sulfate has been used in alginate based cell delivery vehicles for tissue engineering. The release of calcium and gelling kinetics may also be controlled by using calcium salts with pH dependent solubility and the addition of a slowly acting acid such as D-glucono-δ-lactone (GDL). As the pH changes, calcium ions are released. Also calcium containing liposomes have been used as a controllable alginate gelling system. Alginate gel systems based upon internal gelling may have a more defined and limited supply of gelling ions as opposed to diffusion systems where calcium ions are allowed to diffuse into the alginate solution to give a calcium saturated gel.
Various current methods for manufacturing of alginate gel structures have limitations. Some techniques are only useful to make gels of limited sizes and shapes. Depending of the applications there may problems associated with the control of the gelling kinetics. In some case, undesirable materials are present in gels because such materials are residues and by-products of chemically controlled gelling mechanisms. In some cases, non-physiologic pH values are required for gelling and such conditions may present limitations to the use of such methods. There is therefore a need for other gelling systems and formulations.